Knowledge of the composition and structure of the sea floor in a particular area, or along particular paths on the sea floor, can be extremely valuable information. For instance, knowledge of the soil composition of the sea floor, to include the surface of the sea floor and the upper layer of the sea floor to a depth of approximately three or four feet, can be extremely helpful in selecting sites for underwater structures and determining workable routes for cable and pipe line locations. Historically, penetrometers have long been used to determine the physical characteristics of the sea floor. Early penetrometers were projectile-shaped physical sampling devices which impacted the sea floor, then took a soil sample back to the surface for analysis. More recently, penetrometers have been equipped with instrumentation that measures their velocity and acceleration as they impact the sea floor. In operation, the penetrometer is dropped into the water from a surface vessel or aircraft, and allowed to impact and penetrate the sea floor. As the penetrometer impacts and penetrates the sea floor, data describing its deceleration is recorded. This data is then analyzed to determine the depth of penetration achieved by the penetrometer. In turn, the depth of penetration may be used to generate an accurate characterization of the sea floor at the point of impact.
A number of different penetrometer types have been employed. For example, one type of penetrometer has been adapted to carry an accelerometer and associated recording device. Penetrometers of this type require retrieval prior to analysis of the impact data and retrieval generally necessitates the penetrometer be harnessed during descent. This requirement can effectively limit the accuracy of the information obtained. Furthermore, recovery of this type of penetrometer also involves a significant delay at each drop site. A second type of penetrometer uses fine wires to link an on-board accelerometer to a remote shipboard recorder. For this type penetrometer, retrieval is not required but wire failures have generally limited effectiveness of the design.
A third type of penetrometer relies on the Doppler shift associated with a moving acoustic sound source rather than the response of an accelerometer as described previously. Penetrometers of this third type are generally referred to as acoustic penetrometers. In operation, an acoustic penetrometer carries a fixed frequency acoustic sound source and is dropped into the water from a surface vessel or aircraft. A remote receiver then records the acoustic waveform generated by the acoustic penetrometer as it descends and penetrates the sea floor.
During descent, the acoustic penetrometer undergoes a period of acceleration during which the penetrometer may reach its terminal velocity. In any event, the penetrometer's downward velocity is ended with an abrupt deceleration as the penetrometer impacts and penetrates the sea floor. As the acoustic penetrometer accelerates and decelerates, the acoustic waveform recorded at the remote receiver exhibits a corresponding change in frequency. Specifically, during downward acceleration, the recorded waveform exhibits a gradually decreasing frequency. On the other hand, when the penetrometer impacts the sea floor, the recorded waveform exhibits an abrupt increase in frequency. This effect, caused by the relative motion of the penetrometer with respect to the remote receiver, is known as the Doppler Effect. According to the Doppler Effect, the actual frequency (f.sub.o) of the waveform perceived at the remote receiver is related to the true fixed frequency (f) of the acoustic waveform generated by the penetrometer by: EQU f.sub.o =f(v/(v-v.sub.s))
where v is the speed of sound in sea water, and v.sub.s is the speed of the descending penetrometer.
The relationship between the frequency (f.sub.o) perceived by the remote receiver and the downward velocity of the penetrometer allows the frequency waveform recorded by the remote receiver to be translated into an equivalent velocity waveform. More importantly, the resulting velocity waveform may be further analyzed to determine the distance travelled by the penetrometer during its period of rapid deceleration as it impacts and penetrates the sea floor. In turn, the distance travelled by the penetrometer during the penetration event may be used to generate an accurate characterization of the sea floor at the point of impact.
Various difficulties are, however, inherent in the use of Doppler effect and acoustic penetrometers. As can be readily appreciated, the time between penetrometer release and eventual impact may involve many seconds or minutes. In comparison, the actual penetration event may only last for a fraction of a second. Additionally, the overall frequency shift associated with the descending penetrometer is limited by the eventual speed attained by the penetrometer and is, therefore, modest. As a result, location of the penetration event involves searching a large amount of data and locating the relatively slight frequency shift associated with the penetration event.
Once the penetration event is determined, further difficulties emerge. As previously discussed, the calculation of penetration depth requires that the times associated with the initiation and termination of the penetration event be isolated. Unfortunately, the brief duration and relatively slight frequency shift associated with the penetration event make identification of these two critical times difficult. Identification of the initiation and termination times of the penetration event is further complicated by the inevitable presence of noise in the ocean environment.
A system which analyzes the Doppler effect on the acoustic waveform that results as a penetrometer impacts and penetrates into the sea floor is disclosed in U.S. Pat. No. 4,007,633 which was issued to Thompson for an invention entitled "Method of Determining the Physical Characteristics of a Sea Floor." The Thompson system, however, relies on analog processing of the recorded frequency waveform. As a result, the ability of the Thompson system to first locate and then to analyze the acoustic information associated with the penetration event is limited. In particular, the Thompson system lacks a highly developed ability to identify the initiation and termination of the penetration event.
The present invention recognizes the need for an inexpensive signal processing system which is capable of being used in combination with disposable acoustic penetrometers. In light of the above, it is an object of the present invention to provide a signal processing system that can accurately characterize sea floor composition. Still another object of the present invention is to provide a signal processing system that is relatively immune to acoustic noise. Yet another object of the present invention is to provide a signal processing system that can be used in combination with disposable acoustic penetrometers. It is another object of the present invention to provide a signal processing system that includes an enhanced ability to detect and analyze a penetration event. Still another object of the present invention is to provide a signal processing system that can be used in combination an acoustic penetrometer which is simple to use, relatively easy to manufacture, and comparatively cost effective.